1. Field of the Invention
This invention relates to a long distance optical transmission system in which an erbium-doped fiber amplifier is used, and more particularly to a control signal transmission method and apparatus for a long distance optical transmission system.
2. Description of the Relates Art
Conventionally, in a long distance optical transmission system in which an erbium-doped fiber amplifier is used, it is necessary for a terminal equipment to transmit a control signal to a repeater equipment in order to perform various controls and supervision of the repeater equipment. A transmission method for a control signal is based on optical intensity modulation of an optical signal modulated with an information signal further with the control signal. FIG. 1 shows an example of a terminal equipment. In the terminal equipment shown, laser light generated from optical source (LD) 1 is modulated in accordance with an information signal by optical modulator 4 and is further modulated in accordance with a control signal generated from control signal generating circuit 5 by optical intensity modulator 23. Optical amplifier 10 is used to amplify an optical output level which exhibits a drop. FIG. 2 shows an example of a repeater equipment. An optical signal applied to the repeater equipment is amplified by erbium-doped fiber amplifier (EDF) 12 and branched by optical coupler (CPL) 15. One of the two branched optical signals is generated as an output signal while the other branched optical signal is applied to control circuit 21 after it passes through band pass filter (BPF) 20. Optical signals and electric signals of several components are illustrated in FIG. 3. In particular, with regard to the terminal equipment, (A) of FIG. 3 shows an optical output waveform of a modulated information signal of optical modulator 4, (B) shows an output waveform of a control signal of intensity modulation driving circuit 22, and (K) shows an optical output waveform of optical intensity modulator 23, and with regard to the repeater equipment, (L) of FIG. 3 shows an output waveform of optical-electric converter (O/E converter) 19, and (M) shows a waveform of a control signal from BPF 20.
The prior art described above has the following problems. The prior art has a problem in that, if the modulation degree of a control signal is raised in order to amplitude modulate an information signal with a control signal as seen in FIG. 3, this causes deterioration of the information signal.
The prior art has another problem in that, in order to suppress deterioration of an information signal, the modulation degree must be set low, and consequently, the amplitude of a control signal received by the repeater equipment becomes so small that a sufficient signal to noise ratio of the control signal cannot be obtained.
It is an object of the present invention to provide a method and apparatus by which a control signal having a high signal to noise ratio is transmitted without providing any deterioration in characteristic to the transmission signal by modulating a polarization state of an optical signal with the control signal and transmitting the modulated optical signal.
According to an aspect of the present invention, there is provided a control signal transmission method for an optical transmission system, characterized in that an optical signal modulated with an information signal is polarization modulated with a control signal, and the polarization state of the polarization modulation optical signal is varied in synchronism with the control signal to obtain a modulated optical signal and the control signal is transmitted with the modulated optical signal.
According to another aspect of the present invention, there is provided a control signal reception method for an optical transmission system, characterized in that a modulated optical signal produced by polarization modulating an optical signal modulated with an information signal with a control signal and varying the polarization state of the polarization modulated optical signal in synchronism with the control signal is received, and the polarization modulated control signal is extracted by means of a polarizer.
According to a further aspect of the present invention, there is provided a control signal transmission apparatus for a transmission terminal or a repeater equipment of an optical transmission system which multiplexes a modulated optical signal with a control signal, characterized in that it comprises a first polarization modulator for polarization modulating an optical multiplex signal generated from an optical coupler, which multiplexes a plurality of optical signals modulated with information signals, with a control signal, and a second polarization modulator for varying the polarization state of the polarization modulated optical signals in synchronism with the control signal and transmitting resulting modulated optical signals.
According to a still further aspect of the present invention, there is provided a control signal transmission apparatus for a reception terminal or a repeater equipment of an optical transmission system which multiplexes a modulated optical signal with a control signal, characterized in that it comprises a polarizer which applies a received modulated optical signal and extracts a polarization modulated control signal from the received modulated optical signal, and the optical signal obtained by the polarizer is converted into an electric signal and the control signal is generated through a band-pass filter.
The first advantage of the present invention is that a control signal can be superposed and transmitted together with an information signal without any characteristic deterioration of the information signal. This is because, since the polarization state of an optical signal is modulated with the control signal, the intensity of the optical signal is not varied.
The second advantage of the present invention is that a repeater equipment can receive a control signal of a high signal to noise ratio by means of a simple circuit formed by adding only a polarizer to a conventional circuit. This originates from the control of the present invention that the polarization modulation state of the optical signal is made coincide with the polarization surface of the polarizer by the second polarization modulator so that the output amplitude of the control signal may be maximized.